Many waste heat recovery systems employ an intermediate heat transfer fluid to transfer heat from waste heat gases, such as the exhaust gases of a gas turbine, or waste heat gases from industrial processes in stacks to a power producing organic Rankine cycle (ORC) system. One of these waste heat recovery systems is disclosed in U.S. Pat. No. 6,571,548, for which the intermediate heat transfer fluid is pressurized water. Further waste heat recovery systems are disclosed in U.S. patent application Ser. No. 11/261,473 and U.S. patent application Ser. No. 11/754,628, the disclosures of which are hereby incorporated by reference, in which intermediate heat transfer fluids are used from which power can also be produced.
The thermal efficiency of such a prior art waste heat recovery system is reduced due to the presence of the intermediate heat transfer fluid. Furthermore, the capital and operating costs associated with the intermediate fluid system are relatively high.
It would therefore be desirable to obviate the need of an intermediate fluid system by providing a direct heating organic Rankine cycle, i.e. one in which heat is transferred from waste heat gases to the motive fluid without any intermediate fluid circuit. However, a directly heated organic motive fluid achieves higher temperatures than one in heat exchanger relation with an intermediate fluid, and therefore suffers a risk of degradation when brought to heat exchanger relation with waste heat gases and heated thereby as well as a risk of ignition if the organic motive fluid leaks out of e.g. a heat exchanger.
It is an object of the present invention to provide a waste heat recovery system based on a direct heating organic Rankine cycle.
It is an additional object of the present invention to provide a direct heating organic Rankine cycle which safely, reliably and efficiently extracts the heat content of waste heat gases to produce power.
Other objects and advantages of the invention will become apparent as the description proceeds.